1. Field of the Invention
This invention relates to a scanning apparatus which uses a semiconductor laser array as a light source unit.
2. Description of the Prior Art
In prior art, as shown in Japanese Laid-open Patent Application No. 158251/1979, assuring dense scanning-line-pitch with the use of a so-called semiconductor laser array having a plurality of light output portions arranged side by side requires that the direction of the semiconductor laser array is inclined not in a direction orthogonal to the scanning line but obliquely with respect thereto. Each light flux from such a semiconductor laser array diverges with respect to a light ray (central light ray) travelling in a direction parallel to the normal to the end surface of the array.
FIG. 1 of the accompanying drawings shows an example of the plural-beam scanning apparatus according to the prior art. Central light rays ha and hb are emitted parallel to the optical axis g of a condenser lens 2 from a plurality of light output portions 1a and 1b provided in a semiconductor laser device 1. These central light rays pass through the focus F of the condenser lens 2 and pass through a cylindrical lens 3, and thereafter reach the deflecting mirror surface 4a of a deflector 4. At this time, the central light rays ha and hb from the light output portions 1a and 1b are reflected in the deflected direction of the light flux by the deflecting mirror at positions spaced apart from each other on the deflecting mirror surface 4a.
The cylindrical lens 3 causes the light fluxes emitted from the light output portions 1a and 1b and passed through the condenser lens 2 to be imaged as linear images near the deflecting mirror surface 4a. The light fluxes reflected by the deflecting mirror surface 4a are imaged on the surface of a medium to be scanned 6 such as a photosensitive medium by an anamorphic scanning lens system 5.
FIG. 2 of the accompanying drawings shows the manner of imaging in a plane orthogonal to FIG. 1, and use is made of an anamorphic scanning lens which renders a point near the deflecting mirror surface 4a and a point on the surface of the photosensitive medium conjugate. The anamorphic scanning lens 5 comprises, for example, a spherical lens 5a and a toric lens 5b.
The central light rays ha and hb emitted from the light output portions 1a and 1b of the semiconductor laser device enter the positions spaced apart from each other on the deflecting mirror surface 4a, as previously described. That is, as regards the central positions of the linear images formed by the cylindrical lens 3, the central position of only the linear image corresponding to any one of the plurality of light output portions can be installed at a desired position, but the central positions of the other linear images are spaced apart from the desired position. The optical path indicated by broken lines in FIG. 2 shows the manner of imaging in this case, and the central position P of the deviated linear image deviates from the desired position and is not imaged on the surface of the medium to be scanned 6 by the lens system 5 but is imaged at a point P' spaced apart from the surface of the medium to be scanned and thus, so-called defocus occurs. When the deflecting mirror 4 is rotated, the amount of defocus thereof becomes remarkably greater and the imaged spot on the surface of the medium to be scanned becomes large.
To overcome this difficulty, the present applicant has proposed, in U.S. Pat. No. 4,474,422 (issued Oct. 2, 1984), to set the position of the exit pupil of a collimating portion for collimating the light flux from the light source closely adjacent to the deflecting-reflecting surface of the deflector.